Here's a breakdown:

* Gram staining is a technique used to classify bacteria based on their cell wall composition.

* Gram-positive bacteria have a thick layer of peptidoglycan in their cell wall, which traps the crystal violet dye used in the staining process. This results in the bacteria appearing purple under a microscope.

* Gram-negative bacteria have a thinner layer of peptidoglycan and an additional outer membrane that contains lipopolysaccharide (LPS). The outer membrane prevents the crystal violet dye from being retained, so these bacteria appear pink after a counterstain is applied.

Therefore, E. coli is gram-negative because it has a thin peptidoglycan layer and an outer membrane that prevents it from retaining the crystal violet dye during the Gram staining process.

Here are some additional points:

* The outer membrane in gram-negative bacteria provides protection against certain antibiotics and the host's immune system.

* LPS, a major component of the outer membrane, is a powerful endotoxin that can cause serious health problems if released into the bloodstream.

Understanding the differences in cell wall structure between gram-positive and gram-negative bacteria is crucial in understanding their different characteristics and their interactions with the environment and human hosts.